The well was drilled with sea water and the formation is soft clay formation. Then, the bit was totally plugged. We did not have any chance to displace well with water based mud prior to coming out of the hole. We flow checked and came out of hole to change the bit.
Because the formation is clay which will react with seawater resulting formation swelling, we need to develop specific rig procedure to deal with when tripping in hole.
What are the foreseeing issues?
• Clay formation swell
• Pack off due to formation instability
• Soft formation tends to get side tracked easily with mud motor
What is the objective?
• Attempt to condition the well before running back in hole with motor assembly to minimize chance of side track.
These are out rig specific procedures developed to prevent foreseeing problems.
• Run in hole with BHA (bit + DP)
• Slowly trip in hole
• Closely watch drag
• Do not turn the pump on because of soft formation which can be easily side track
• Once tripping in hole to the previous TD, stage pump up slowly and watch for return. The well has a potential to collapse and pack off.
• If the pack off is seen, stop pump and work down string with rotation to free the pack off. Do not apply pressure if the pack off is occurred.
• Attempt to establish circulation to full drilling rate and closely watch returns
• Slowly work pipe and rotation while circulating
• Circulate with sea water and check pick up / slack off weight to ensure hole is clean
• Pump hi-vis sweep and displace well with water base mud
• Circulate hole clean and monitor weight
• Once the well is clean, trip out of hole to change BHA
• Make up BHA (mud motor)
• Trip in hole to casing shoe
• Orient tool phase to the original direction
• Trip in hole without rotation and pump to TD
With these procedures, we were able to get back to bottom, minimize risk and safely resume drilling operation.
We wish this lesson learned would help you in the future if you see the similar issue.
Thanks for this good practice, I was working in this type of formation last month. some additional recommendation:
In soft clay and tripping operation, if swelling is important, potential to swab formation is high while tripping out due under-gauge diameter, in case permeable interbedded formation in claystone section reconsider lower tripping speed.
As mentioned, it s better to not pump down to avoid “side tracking”, however soft reactive claystone tends to bit balling. to avoid tag bottom with bit balled up, its generally recommended to keep high HSI (2-3) and at least center nozzle in case roller cone to help clean the bit.
While drilling this kind of formation, pickup slack off, drilling parameters generally did not indicate any well-bore problem, but all coming when tripping out. Short trip is sometime good to know where we stand.
WHILE DRILLING THROUGH A SOFT CLAY FORMATION WHY WOULD YOU DISPLACE THE HOLE WITH FRESH WATER ? I THOUGHT THAT SALT WATER WAS MORE INHIBITIVE THAN FRESH WATER, THERE FORE THE CLAY FORMATION WOULD BE LESS REACTIVE TO SWELLING.
IN WEST TX. WELL USE FRESH WATER FOR SPUD MUD/ 3% OIL ON SURFACE HOLE AND BAKERSFIELD, CA. WE USED FRESH WATER WITH WATER LOSS ADDITIVE / 5% WTR LOSS.
JUST CURIOS WHY YOU FILL THE HOLE WITH FRESH.
This was a good one.I have just encounted this type of formation in one well we did and this procedure was respected though there was alot of gas and this pushed us to go to about 14.3ppg mud weight to finally plug the well.
Thanks for this vital information.